A phase change memory device is used for data storing. The phase change memory device may switch between two states, namely a set state and a reset state. In the set state information is stored by the phase change memory device, whereas in the reset state the phase change memory device is cleared for a further storing of information.
For this purpose, the phase change memory device comprises a phase change material, e.g. a composition of a germanium-antimony-tellurium phase change material, which is bridged between two electrodes. One of the electrodes is electrically coupled to a switching element for switching the phase change memory device. The switching element is further connected to a current source. The second electrode is electrically connected to a current or voltage sensing device, which in turn is electrically connected to a ground potential.
Conventionally, the switching element is designed as a semiconductor transistor, e.g. a complementary metal-oxide-semiconductor transistor (CMOS).
Upon switching the transistor, a current is supplied to the electrode which is connected to the transistor. The induced current flows from the one electrode being connected to the transistor through the phase change material to the other electrode such that a heat induced change in the crystal structure of the phase change material occurs. Upon changing the crystal structure of the phase change material other physical properties of the phase change material, in particular the resistivity, also changes.
This change in the crystal structure of the phase change material represents a switch of the phase change memory device from the reset state to the set state or vice-versa. The phase change material thus reversibly undergoes a crystal structure change from the amorphous state to the crystalline state or vice versa. A change from the crystalline state to the amorphous state of the phase change material is caused by a “reset” current of a short duration and a high magnitude. A change from the amorphous state to the crystalline state of the phase change material is caused by a “set” current of a long duration and a low magnitude.
Upon reading the information stored by the phase change memory device, a small current is supplied to the one electrode being connected to the transistor such that the phase change material does not show a change in its crystal structure. The current or voltage sensing device detects the current flowing through the phase change material.
In order to switch the phase change memory device in a very efficient way, it is desirable that the available power provided by the transistor for the switching event will be deposited in the phase change material. When using a CMOS-transistor as the switching device, a relatively low power is available for the switching event, as the maximum current and voltage output of the transistor is lowered compared to a conventional semiconductor transistor. In turn, a high resistivity of the phase change material is desirable for compensating the transistor performance. Thus it is highly desirable to adjust the resistivity of the phase change material.
It is known from US 2008/0023686 A1 that physical properties such as the refractive index and the resistivity of a Ge2Sb2Te5 phase change material can be modified by doping indium. Possible doped phase change materials may comprise a germanium concentration of about 10 at % to about 25 at %, an antimony concentration of about 15 at % to about 30 at %, a tellurium concentration of about 40 at % to about 70 at %, and an indium concentration of about 5 at % to about 15 at %.
However, only a small impact on the physical properties of the phase change material may be achieved by indium doping of the phase change material.